OBSERVING TRANSITS WITH JWST:
SOME OPERATIONAL ISSUES
Kailash C. Sahu
STScI
5/28/2016
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OUTLINE
 Science Cases for Transit Observations
 Observation scenarios (NIRCam, NIRSPEC and
MIRI)
 Saturations/Expected data volumes…
 Possible solutions
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SCIENCE CASES
I.
Science Case - I: An Earth-like planet
around a nearby sun-like star

Assume: every star has an Earth-like planet
•
The probability of transit for an Earth at 1 AU
around a G-type star: ~ R⨀/a ~ 7 x 1010/1.5
x 1013 ~ 0.5%
The optimal sample size needed to observe
the first earth-like planet around a sunlike star ~200
Expected brightness of that the first sun-like
host of an earth-like planet: V ~6.
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Science Case I: Observations
Transit duration for an earth analogue ~ 12
hours
Expected Science observation:
•
Continuous monitoring of the star
before, during, and after transit (total of
36 hours)
— Imaging with NIRCam: to get very
high S/N for (i) accurate radius
determination, (ii) determination of
inclination angle…
— Spectroscopy with NIRSpec: high
S/N spectra to detect possible
atmospheric features
— Imaging and spectroscopy with
MIRI
— Imaging with FGS/TFI.
P ~ 1yr
Ttr~12 hrs
NIRCam
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 2 Modules
 Each module has two channels (SW:0.6
to 2.3 m & LW:2.4 to 5 m)
 Total of 10 detectors, 8 for SW
and 2 for LW
 Each detector has 2048x2048
pixels
Pixel scale:
SW: 0.032”/pix; LW: 0.064”/pix
 Image size: 2.2’ x 4.4’. SW and
LW channels observe the same
field simultaneously
Module B

Module A
Short wavelength channel
2.2’
Long wavelength channel
SCIENCE CASE-I: Expected Data Rate
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Can we observe such a bright star?
•
•
•
Saturation occurs at V~17, for the minimum ‘exp time’ of 10.6sec.
Fortunately, NIRCam has defocusing mirrors, which allow observations of stars up to V ~4.
Subarrays can also be used which allow shorter integrations, and allow observations of
brighter stars.
Courtesy John Krist
In Focus F210M
12l Defocus x10
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SCIENCE CASE-I: Expected Data Rate
Expected observation cadence:
NIRCam: 10.6 sec + 10.6 for readout, 2 detectors (1 SW and 1 LW)
(MULTIACCUM pattern:
TGROUP=10.6 s, NGROUP=1 to 2, NFRAME=1, NSKIP=0)
(Data volume is roughly the same if subarrays are used for brighter stars)
Expected Data Volume per day:
2(channels)x2048x2048(pixels)x16(bits per pixel read) x86400/20.6 = 5.6e11 = 563 Gbits/day.


This exceeds the data volume limit by a factor of ~2.
Compression algorithm will help, but may not completely solve the problem.

For NIRSPEC (which has 2 detectors), the data volume constraints are
similar.
For MIRI (one detector), constraints are smaller by a factor of 2.

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SCIENCE CASES
Science Case - II: Determining the
frequency of hot earths
(Or, to detect the first extragalactic exoplanets)
 The goal is to determine the
frequency of hot earths
 Expected transit signal ~ 0.1% (R ~
3 REarth), transit duration ~ 3 hours,
orbital period ~ 1 to 5 days.
 A reasonable way to achieve this is
to monitor a rich stellar field, similar
to the SWEEPS program towards
the Galactic bulge.
HST Image of
the SWEEPS Field
2.3’ x 2.3’
~200,000 stars
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Determining the frequency of
hot earths
POSSIBLE TARGET:

Monitor a nearby, rich, high-metallicity cluster, such as NGC 6791
([Fe/H] ~+0.4).

Saturation will be just avoided for solar-like star with V ~17.

This coincides with the turn-off magnitude for this cluster, making this
an ideal target.

Hot-earths can be detected with 10-sigma detection.

Monitoring of a 2000 to 5000 stars can lead to detection of ~20 hot
earths, further boosted by metallicity.
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SCIENCE CASE-II: Observations
Expected Observations:

NIRCam imaging using all the 10
detectors

Continuous monitoring for 8 to 10 days
similar to SWEEPS and 47-TUC HST
observations.

Filters to be used: F115W and F150W
for the SW channel; F277W and F356W
for the LW channel
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Extra-galactic planets:


Stars in LMC are ~3 magnitudes
fainter than the bulge stars.
NIRCAM/JWST is more sensitive
by 2 to 3 mag. compared to
ACS/HST.
There are 100,000 stars in the
NIRCAM/JWST calibration field,
which is ideal for such a study.
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JWST Calibration Field
Courtesy: Jay Anderson
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SCIENCE CASE-II: Expected Data Rate
Expected observation cadence:
10.6 sec + 10.6 for readout
(MULTIACCUM pattern:
TGROUP=10.6 s, NGROUP=1 to 2, NFRAME=1, NSKIP=0)
Expected Data Volume per day:
10(channels)x2048x2048(pixels)x16(bits per pixel read) x86400/20.6 =
2.8e12 = 2,815 Gbits/day.


This exceeds the data volume limit by an order of magnitude!
One way to solve this impasse would be to require for this type of
observation using exposures 10 times as long, or stars 2.5 magnitudes
fainter. This, however, results in a less interesting experiment. Being able to
reach to a few Earth radii as the limit for planet size would certainly be
advantageous. And important spectroscopic follow-up observations are also
possible at V ~ 17, but impractical at V > 20.
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Science Case I: Expected Data Rate
Possible solution:
Fortunately, the transits typically last 1 to 12
hours.
So it would be scientifically acceptable to
average, or sum the individual 10s
exposures to 10 minute cadence
onboard, which provides a clean
solution.
FPAP has the capability to do such onboard averaging from 2 to 16, in
powers of 2. It can handle full frames
from all the 10 NIRCam detectors.
The plan is to take advantage of this
capability, which will facilitate these
transit observations.
P ~ 1yr
Ttr~12 hrs
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NIRSPEC
(with thanks to: Jason Tumlinson)
 Wavelength range: 0.6 to 5 microns.
 3 observing modes: R ~ 100 prism mode, R
~ 1000 multi-object mode, and R ~ 3000
integral field and long-slit spectroscopy
mode.
 Two 2048 x 2048 detectors
 A 1.6x1.6 arcsec slit has been specially
introduced in the MSA for exoplanet transit
observations.
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NIRSPEC
(with thanks to: Jason Tumlinson)
SATURATION:
 >85% of the planet-hosting stars are too bight in full-frame mode.
 Subarrays allow observations of ~99% of the planet hosts.
 Subarrays restricted to spectral features will further facilitate such observations.
 On-board averaging capability can solve any data-volume problems.
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MIRI
thanks to: Scott Friedman
 Wavelength range: 5 to 27 microns.
 Imager: broad and narrow-band
imaging, phase-mask
coronagraphy, Lyot coronagraphy,
and prism low-resolution (R ~
100) slit spectroscopy from 5 to
10 microns, 1024 x 1024
detector
 Spectrograph: R~300, over 5 to
27 microns, 1024 x 1024 detector.
 Maximum data volumes: ~2 times
larger than the data volume limit,
which can be solved by on-board
averaging.